GPCRs Explained: How Peptide Receptors Work

Almost every peptide in our research library works by knocking on a door: a receptor on the surface of a cell. For most of them, that door belongs to one family, the G protein-coupled receptors, or GPCRs. Understanding how a GPCR works explains why peptides can be so selective, why some effects fade with repeated exposure, and why about a third of all approved drugs act on this single receptor family.

This guide explains what GPCRs are, how they turn a signal outside the cell into a response inside it, the main receptor classes, and which receptors the peptides in our catalogue act on.

The short answer

• GPCRs are cell-surface receptors that cross the cell membrane seven times. When a signalling molecule binds outside, the receptor changes shape and activates a G protein inside the cell.
• Humans have roughly 800 GPCRs, about half of them smell receptors.
• They are the most heavily targeted receptor family in medicine: a 2017 analysis found that about 34% of FDA-approved drugs act on GPCRs.
• Most peptide hormones and neuropeptides use GPCRs, including GHRH, ghrelin, GLP-1, VIP, kisspeptin, oxytocin and the opioid peptides.
• Not every peptide works this way. Insulin uses an enzyme-linked receptor, growth hormone a cytokine receptor, and some research peptides have no confirmed receptor at all.

What a GPCR is

A G protein-coupled receptor is a single protein chain that weaves back and forth through the cell membrane seven times, forming a bundle of seven helices. The outside part, together with the top of the bundle, forms the binding site for the signalling molecule. The inside part connects to a G protein, a three-part switch made of alpha, beta and gamma subunits that sits just under the membrane.

From signal to response

1. BindingA peptide fits into the receptor’s binding pocket, much like a key in a lock.
2. Shape changeThe seven helices shift, opening a cavity on the inside face of the receptor.
3. G protein switchThe G protein’s alpha subunit swaps GDP for GTP, switching it on, and separates from the beta-gamma pair.
4. AmplificationThe activated subunits turn on enzymes such as adenylyl cyclase, which makes cyclic AMP, or phospholipase C, which releases calcium. One receptor can generate thousands of messenger molecules.
5. Switch-offThe alpha subunit slowly breaks down its GTP and switches off. Meanwhile, receptor kinases tag the active receptor, β-arrestin binds it, and the receptor is pulled into the cell. This is why responses can fade with repeated stimulation.

The work that mapped this machinery earned Robert Lefkowitz and Brian Kobilka the 2012 Nobel Prize in Chemistry.

The GPCR classes

Class B is especially important for peptide research: its receptors bind longer hormones in a two-step grip, with a large outer domain catching the hormone’s tail before its start slides into the core. Our VIP guide shows how closely related the class B hormones are in sequence.

Our peptides and their receptors

That last row matters. When no receptor has been identified, mechanisms are inferred from downstream effects, which makes results harder to interpret and replicate. A known receptor is one of the strongest signs that a peptide’s biology is well understood.

Why receptor biology matters in research

• Selectivity. Two peptides can share a receptor and still behave differently. GHRP-6 and ipamorelin both activate the ghrelin receptor, but in the study that introduced ipamorelin, only GHRP-6 also raised ACTH and cortisol.
• Desensitization. Continuous stimulation can switch a receptor system off. The classic example is GnRH: natural pulses stimulate the reproductive axis, but constant exposure to a GnRH agonist suppresses it, which is exactly how several prescription drugs work.
• Biased signalling. Some ligands favour the G protein route over the β-arrestin route, or the reverse. Drug designers now exploit that to separate wanted effects from unwanted ones.
• Species differences. Receptors vary between species, so a peptide’s potency in rodent cells is not always a guide to human cells.

For the chemistry behind the peptides themselves, start with our guide what are peptides?, and for why many peptide drugs need chemical modification, see how peptides are made to last.

Frequently asked questions